It is known to provide offshore surface installations, such as drilling rigs used in connection with subsea well operations, with heave compensators. A heave compensator is arranged in the interface between a pipe string extending down to the sea floor and the surface installation. When the surface installation is moved vertically due to waves and swells, the heave compensator ensures that the vertical movement of the installation is not transferred to the pipe string, keeping the pipe string vertically still with regard to the sea floor. However, if the pipe string is disconnected from a sea floor facility, such as a subsea well installation, the pipe string will exhibit some vertical movements despite the heave compensator. For instance, with vertical heave movements of the surface installation in the order of 7 meters, the pipe string will perhaps move vertically up and down a distance in the order of about 30 cm.
Due to the heave compensator, the surface installation can keep operating even in quite rough sea with waves being several meters high. However, if the conditions become too extreme, operation must be halted and the surface installation must be disconnected from the pipe string extending into the well.
For instance, when drilling a subsea well from a floating drilling rig, the drill pipe can extend several thousands of meters down into the well. In order to disconnect the drill pipe from the drilling rig, the drill pipe is hung off at the top of the well. To do this, the drill pipe is first pulled up to the drilling rig with a distance approximately corresponding to the sea depth. Then a hang-off tool is connected to the drill pipe below it and lowered down to the top of the well on a drill pipe connected to the upper part of the tool. With the hang-off tool, the drill pipe is hung off at the top of the well, for instance in the well head, the tree, or bore protector, while extending into the drilled well. The hang-off tool is then disconnected from the drill pipe above it, on which it was lowered, thus becoming disconnected from the drilling rig.
When surface conditions have returned to operating conditions, the drilling rig can again be connected to the hang-off tool. A suspension element, such as a drill pipe, is lowered down from the installation and connects to the hang-off tool. The hang-off tool is then pulled up to the rig and removed. Then drill pipe is again extended and lowered, and operations can be resumed.
When disconnecting the hang-off tool from the suspension element, such as the pipe string above it, the suspension element will exhibit some vertical movement despite the heave compensator at the drilling rig if considerable waves affect the rig. Thus, soon after disconnection, the pipe string can move down and collide with the part from which it was disconnected. Due to the weight of the suspension elements this can cause substantial damage to both connection interfaces. That is, both the lower and upper part of the mating connection parts can be damaged. The same problem arises when the parts are to be reconnected. As the upper part closes in on the lower part when being lowered from the surface, it can in addition to the intentional lowering exhibit reciprocating vertical movements. Before the upper connection part is properly connected to the lower part, the connection interface may thus be damaged.
Conventional means of connection are threads. Thus, when connecting or disconnecting, the upper connection part is rotated as it is lowered onto or pulled upwardly from the lower part. The outermost threads are therefore often damaged. An obvious means to overcome this problem would be to use larger threads that tolerate larger impacts. Larger threads would however imply larger thread pitch, which may increase the risk of the connection unscrewing itself. Furthermore, even larger threads could be damaged severely enough to cause problems when connecting or disconnecting. In any case, the vertical heave movements will cause undesirable tear and wear.
Also other means for releasable connection may be used. However, regardless of which type of connection being used, a downwardly moving suspension element in the form of a string of pipe represents substantial forces that in any case are desirable to reduce.
Thus, the object of the present invention is to provide a solution to the above-mentioned problem of damage to the connection interface when disconnecting and reconnecting.